In this specification where a document, act or item of knowledge is referred to or discussed, this reference or discussion is not an admission that the document, act or item of knowledge or any combination thereof was at the priority date, publicly available, known to the public, part of common general knowledge, or otherwise constitutes prior art under the applicable statutory provisions; or is known to be relevant to an attempt to solve any problem with which this specification is concerned.
Sunlight concentrators may include optical systems which possess entrance apertures larger than their exit apertures designed to concentrate sunlight onto a target area smaller than that of the entrance aperture area. The “aperture,” a term of art, refers to the effective area that gathers or collects incident energy or sunlight, and the effective area that receives concentrated energy or sunlight thereon. The ratio of the area of the entrance aperture, Aa, to the area of the target aperture At, defines the system's geometric concentration ratio, Cg, i.e., Cg=Aa/At. Historically, sunlight concentrators have been realized using geometric optical elements having feature sizes much greater than the wavelength of light and operate under the principles of geometric or ray optics. These optical elements are either reflective or refractive in nature.
Reflective sunlight concentrators are typically formed from mirrors having curved surfaces (e.g., spherically curved or parabolically curved), such as the one-stage, three-dimensional (i.e., point focus) coaxial elliptic parabolic concentrator, which consists of a mirror formed in the shape of a paraboloid of revolution, i.e., the curvature of the mirror is described by (x/a)2+(y/b)2−z=0; where a=b, or the two-dimensional (i.e., line-focus) coaxial parabolic linear concentrator, which consists of a mirror which has the shape of a two-dimensional cross-section of a paraboloid of revolution, cut along the z axis and extruded longitudinally to form a trough. Arrays of planar, i.e., flat mirrors are sometimes used to approximate curved mirrors.
Refractive concentrators generally rely upon spherical or aspherical lenses or approximations thereof. As shown in FIGS. 1-2A, these lenses are either conventional curved lenses 10, or stepped or Fresnel lenses 20. These lenses are used to focus or concentrate sunlight 30 to either a point or a line. Conventional lenses consist of a solid, optically transparent medium with an index of refraction greater than that of air (i.e., n>1.0003). Typically, materials such as fused silica glass (n=1.459) or polycarbonate plastic, such as Lexan (n=1.586) are utilized. Stepped or Fresnel lenses have a reduced thickness compared to that of a conventional lens. In traditional imaging Fresnel lenses 20 the reduction in thickness is achieved by dividing the surface of a standard lens, which is continuous, into a set of discontinuous prismatic surfaces, each of which has the same curvature as the segment of the conventional lens surface it approximates. Stepped or Fresnel lenses are typically fabricated using the same materials as conventional lenses.
Curved mirrors, arrays of planar mirrors which are used to approximate a curved mirror, and lenses operate by way of geometric optical principles. Curved mirrors have a focal length, f, which depends upon the radius of curvature of their surfaces, typically f is of one-half of the radius of curvature. Arrays of planar mirrors have an effective focal length which depends upon the radius of the virtual curve formed by their physical placement relative to one another. Conventional lenses have a focal length dependent upon the radius of curvature of their entrance and exit surfaces, the refractive index n of the lens material, and the physical thickness of the lens material.
In addition, concentrated solar energy impinging upon solar arrays, such as arrays of photovoltaic cells, can cause catastrophic and irreversible damage to the cells due to overheating should the requisite array cooling system fail.